Method and apparatus to control delivery of high-voltage and anti-tachy pacing therapy in an implantable medical device

ABSTRACT

A method and device for delivering a therapy in response to detection of abnormal cardiac rhythms that includes delivering a first therapy during a first delivery period, substantially simultaneous with coupling of a charging circuit and an energy storage device to generate stored energy on the energy storage device. A determination is made as to whether to deliver the first therapy during a second delivery period subsequent to the first delivery period in response to the predetermined level of stored energy not being generated on the energy storage device. The device then delivers the first therapy during a third delivery period subsequent to the second delivery period in response to the first therapy not being delivered during the second delivery period and the predetermined level of stored energy not being generated on the energy storage device.

FIELD OF THE INVENTION

The present invention relates generally to implantable medical devices;and, more particularly, to reducing power consumption in an implantablemedical device.

BACKGROUND OF THE INVENTION

Implantable cardioverter-defibrillator (ICD) art has long distinguishedventricular tachyarrhythmias by rate and type. Ventricular tachycardias(VTs), which generally include arrhythmias having rates between 150 and250 bpm or more, can be further differentiated by their ECGconfiguration as either monomorphic or polymorphic. Arrhythmias withrates above an upper VT range, and up to approximately 350 bpm, areoften termed ventricular flutter waves. Chaotic waveforms at rateshigher than 350 bpm are classified as ventricular fibrillation (VF).

To treat each type of arrhythmia with an appropriate therapy, ICDs havebeen equipped with “tiered therapies”. Such devices are generallyreferred to as Pacer-Cardioverter-Defibrillators (PCDs). PCDs generallydifferentiate arrhythmias by rates, with programmable therapies to treata respective type of detected arrhythmia(s). In such devices, theless-dangerous arrhythmias such as VT are treated by delivering a seriesof low-power pacing pulses to the heart at a relatively high rate. Thistherapy is often referred to as anti-tachyarrhythmia pacing therapy(ATP). In contrast, more perilous arrhythmias such as VF are oftentreated using a more aggressive shock therapy. For example, many PCDsmay be programmed to first treat a VT with low-power ATP and then, ifthe VT progresses to ventricular flutter or fibrillation, deliver one ormore high-power cardioversion or defibrillation shocks.

Many implantable anti-tachycardia pacemakers have the capability ofproviding a variety of anti-tachycardia pacing regimens. Normally, theseregimens are applied according to a pre-programmed sequence, such asburst or ramp therapies among others. Each therapy extends over a seriesof a predetermined number of pacing pulses. After the series of pacingpulses is delivered, the devices check to determine whether the seriesof pulses was effective in terminating the detected tachyarrhythmia.Termination is generally confirmed by a return to sinus rhythm, forexample, identified by a sequence of a predetermined number ofspontaneous depolarizations separated by greater than a definedinterval. In the absence of detected termination, the PCD applies moreaggressive therapies such as synchronized cardioversion shocks ordefibrillation shocks. While the delivery of ATP in some cases makesshock therapy unnecessary, a further reduction in the frequency of shockdelivery is still desirable.

Applying an electrical pulse to the heart, whether a pacing pulse or ashock, requires charging of one or more output capacitors. Generally,the amount of energy required to delivery pacing pulses is low. Thistype of therapy may therefore be delivered by a low-power output circuitrelatively instantaneously. On the other hand, high-power shocks requirea set of high-voltage capacitors that may require several seconds toreach a fully-programmed charge. As stated above, when a tiered therapyapproach is utilized, both of these therapies may be used to “break” thetachyarrhythmia. That is, first ATP is delivered. During this time, thehigh-voltage capacitors may be charged so that if ATP fails to break theVT, a high-voltage shock may be delivered soon thereafter. If the VT isterminated by ATP, the charged high-voltage capacitors must abortdelivery and internally “leak off” the stored energy in the capacitors,which depletes battery power. This can significantly shorten the usefullife of the implanted device.

What is needed, therefore, is a method and apparatus to deliversuccessful ATP therapy without needlessly depleting battery resources.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present invention will be readily appreciated as theybecome better understood by reference to the following detaileddescription when considered in connection with the accompanyingdrawings, wherein:

FIG. 1 is a block diagram of an illustrative embodiment of animplantable medical device in which the present invention may beemployed;

FIGS. 2A and 2B are exemplary timing diagrams illustrating ananti-tachyarrhythmia pacing therapy during capacitor charging (ATP-DCC)mode of an implantable medical device according to the presentinvention;

FIG. 3 is an exemplary timing diagram illustrating ananti-tachyarrhythmia pacing therapy before capacitor charging (ATP-BCC)mode of an implantable medical device according to the presentinvention;

FIG. 4 is an exemplary timing diagram illustrating an ongoing VT episodethat fails to break following ATP-BCC therapy;

FIG. 5 is a state diagram illustrating transitions between therapymodes, according to the present invention;

FIG. 6A is a flowchart of operation of an implantable medical device inan ATP-DCC mode, according to an embodiment of the present invention;

FIG. 6B is a flowchart of operation of an implantable medical device inan ATP-BCC mode, according to an embodiment of the present invention;

FIG. 6C is a flowchart of operation of an implantable medical device ina best available therapy mode, according to an embodiment of the presentinvention;

FIG. 6D is a flowchart of operation of an implantable medical device ina best available therapy mode, according to an embodiment of the presentinvention;

FIG. 7 is a flowchart illustrating a mode switch according to anembodiment of the present invention; and

FIG. 8 is a flowchart illustrating a mode switch according to anembodiment of the present invention.

DETAILED DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of an illustrative embodiment of animplantable medical device in which the present invention may beemployed. As illustrated in FIG. 1, the device is embodied as amicroprocessor based stimulator. However, other digital circuitryembodiments and analog circuitry embodiments are also believed to bewithin the scope of the invention. For example, devices having generalstructures as illustrated in U.S. Pat. No. 5,251,624 issued to Bocek etal., U.S. Pat. No. 5,209,229 issued to Gilli, U.S. Pat. No. 4,407,288,issued to Langer et al, U.S. Pat. No. 5,662,688, issued to Haefner etal., U.S. Pat. No. 5,855,593, issued to Olson et al., U.S. Pat. No.4,821,723, issued to Baker et al. or U.S. Pat. No. 4,967,747, issued toCarroll et al., all incorporated herein by reference in theirentireties, may also be usefully employed in conjunction with thepresent invention. Similarly, while the device of FIG. 1 takes the formof a ventricular pacemaker/cardioverter, the present invention may alsobe usefully employed in a device having atrial pacing and cardioversioncapabilities. FIG. 1 should thus be considered illustrative, rather thanlimiting with regard to the scope of the invention.

The primary elements of the implantable medical device illustrated inFIG. 1 are a microprocessor 100, read-only memory (ROM) 102,random-access memory (RAM) 104, a digital controller 106, an inputamplifier circuit 110, two output circuits 108 and 107, and atelemetry/programming unit 120. Read-only memory 102 stores the basicprogramming for the device, including the primary instruction setdefining the computations performed to derive the various timingintervals employed by the cardioverter. RAM 104 generally serves tostore variable control parameters, such as programmed pacing rate,programmed cardioversion intervals, pulse widths, pulse amplitudes, andso forth which are programmed into the device by the physician.Random-access memory 104 also stores derived values, such as the storedtime intervals separating tachyarrhythmia pulses and the correspondinghigh-rate pacing interval.

Controller 106 performs all of the basic control and timing functions ofthe device. Controller 106 includes at least one programmable timingcounter, which is initiated upon detection of a ventricular activation,and which times intervals thereafter. This counter is used to generatethe basic timing intervals used to deliver anti-tachy pacing (ATP)pulses, and to measure other intervals used within the context of thecurrent invention. On time-out of the pacing escape interval or inresponse to a determination that a cardioversion or defibrillation pulseis to be delivered, controller 106 triggers the appropriate output pulsefrom high-voltage output stage 108, as discussed below.

Following generation of stimulus pulses, controller 106 may be utilizedto generate corresponding interrupts on control bus 132, wakingmicroprocessor 100 from its “sleep” state, allowing microprocessor 100to perform any required mathematical calculations, including alloperations associated with evaluation of return cycle times andselection of anti-tachyarrhythmia therapies according to the presentinvention. The timing/counter circuit in controller 106 also controlstiming intervals such as ventricular refractory periods, as is known inthe art. The time intervals may be determined by programmable valuesstored in RAM 104, or values stored in ROM.

Controller 106 also generates interrupts for microprocessor 100 on theoccurrence of sensed ventricular depolarizations or beats. On occurrenceof a sensed ventricular depolarization, in addition to an interruptindicating its occurrence placed on control bus 132, the then-currentvalue of the timing/counter within controller 106 is placed onto databus 122. This value may be used by microprocessor 100 in determiningwhether a tachyarrhythmia is present, and further, in determining theintervals separating individual tachyarrhythmia beats.

Output stage 108 contains a high-output pulse generator capable ofgenerating shock therapy to be applied to the patient's heart viaelectrodes 134 and 136, which are typically large surface areaelectrodes mounted on or in the heart, or located subcutaneously. Otherelectrode configurations may also be used, including two or moreelectrodes arranged within and around the heart. Typically the highoutput pulse generator includes one or more high-voltage capacitors 109,a charging circuit 111 for transferring energy stored in a battery 115to the high-voltage capacitors 109, an output circuit 113 and a set ofswitches (not shown) to allow delivery of monophasic or biphasiccardioversion or defibrillation pulses to the electrodes employed.

In addition to output circuit 108, output circuit 107 is provided togenerate pacing pulses. This circuit contains a pacing pulse generatorcircuit that is coupled to electrodes 138, 140 and 142, and which areemployed to accomplish cardiac pacing, including ATP pacing pulses, bydelivery of a electrical stimulation between electrode 138 and one ofelectrodes 140 and 142. Electrode 138 is typically located on the distalend of an endocardial lead, and is typically placed in the apex of theright ventricle. Electrode 140 is typically an indifferent electrodemounted on, or adjacent to, the housing of the cardioverterdefibrillator. Electrode 142 may be a ring or coil electrode located onan endocardial lead slightly proximal to the tip electrode 138, or itmay be another electrode positioned inside or outside the heart.Although three electrodes 138-142 are shown in FIG. 1 for deliveringpacing pulses, it is understood that the present invention may bepracticed using any number of electrodes positioned in any pacingelectrode configuration known in the art. Output circuit 108 may becontrolled by control bus 126, which allows the controller 106 todetermine the time, amplitude and pulse width of the pulse to bedelivered. This circuit may also determine which electrode pair will beemployed to deliver the pulse.

Sensing of ventricular depolarizations (beats) is accomplished by inputamplifier 110, which is coupled to electrode 138 and one of electrodes140 and 142. Signals indicating both the occurrence of naturalventricular beats and paced ventricular beats are provided to thecontroller 106 via bus 128. Controller 106 passes data indicative of theoccurrence of such ventricular beats to microprocessor 100 via controlbus 132 in the form of interrupts, which serve to wake up microprocessor100. This allows the microprocessor to perform any necessarycalculations or to update values stored in RAM 104.

Optionally included in the device is one or more physiologic sensors148, which may be any of the various known sensors for use inconjunction with implantable stimulators. For example, sensor 148 may bea hemodynamic sensor such as an impedance sensor as disclosed in U.S.Pat. No. 4,865,036, issued to Chirife or a pressure sensor as disclosedin U.S. Pat. No. 5,330,505, issued to Cohen, both of which areincorporated herein by reference in their entireties. Alternatively,sensor 148 may be a demand sensor for measuring cardiac outputparameters, such as an oxygen saturation sensor disclosed in U.S. Pat.No. 5,176,137, issued to Erickson et al. or a physical activity sensoras disclosed in U.S. Pat. No. 4,428,378, issued to Anderson et al., bothof which are incorporated herein by reference in their entireties.Sensor processing circuitry 146 transforms the sensor output intodigitized values for use in conjunction with detection and treatment ofarrhythmias.

External control of the implanted cardioverter/defibrillator isaccomplished via telemetry/control block 120 that controls communicationbetween the implanted cardioverter/pacemaker and an external device,such as a communication network or an external programmer, for example.Any conventional programming/telemetry circuitry is believed workable inthe context of the present invention. Information entering thecardioverter/pacemaker from the programmer is passed to controller 106via bus 130. Similarly, information from the cardioverter/pacemaker isprovided to the telemetry block 120 via bus 130.

FIGS. 2A and 2B are exemplary timing diagrams illustrating ananti-tachyarrhythmia pacing therapy during capacitor charging (ATP-DCC)mode of an implantable medical device according to the presentinvention. As illustrated in FIG. 2A, after detection of a VT cardiacrhythm 201, microprocessor 100 activates controller 106 to initiate bothcapacitor charging 208 of high voltage capacitor or capacitors 109 viacharging circuit 111 and ATP therapy delivery 204 substantiallysimultaneously at time 202. High-rate VT 201, which in one embodiment isdefined to include rhythms between 185 and 260 beats per minute (bpm),is treated by one sequence of Burst or Ramp or other type ATP-DCCtherapy 204 that extend until an end 207 of a predetermined period oftime 206, or alternatively, for a predetermined number of pacing pulsesending at end time 207. Once delivery of the sequence of ATP-DCC therapyis completed, i.e., at end 207 of predetermined period of time 206,capacitor charging 208 is paused during a redetection or verificationperiod 209 during which a determination is made as to whether the VTrhythm 201 is redetected. In this case, the sequence of ATP-DCC therapycauses the VT rhythm to terminate, or “break”, so that a normal sinusrhythm 210 is resumed. Therefore, capacitor charging 208 remains in apaused state until VT cardiac rhythm 201 is redetected, as will bedescribed in detail below.

As illustrated in FIG. 2B, if VT rhythm 201 is redetected during period209, capacitor charging 208 is resumed once period 209 is completed,i.e., at an end time 211 of time period 209. According to one embodimentof the present invention, a second sequence of ATP-DCC therapy may bedelivered substantially simultaneously with the resumption of capacitorcharging 208 at end time 211 and the process repeated until capacitor109 is completely charged, as will be described in detail below.According to another embodiment of the present invention, illustrated inFIG. 2B, no additional ATP-DCC therapy is delivered and resumption ofcapacitor charging 208 continues until capacitors 109 are charged to adesired charge level at charge time end 212 in preparation for deliveryof a shock, if necessary. A non-committed synchronization period 214begins at charge time end 212. During this synchronization period 214,the patient's cardiac rhythm is evaluated to locate an appropriate timeto deliver a shock and to determine if the VT rhythm is redetected. Theshock will be delivered at the end 216 of the synchronization period 214unless it is determined that the VT episode has terminated. If theepisode has terminated prior to the end 216 of the synchronizationperiod 214, the process charge remains on the capacitors 109 and thedevice continues to monitor for subsequent detected VT rhythms, at whichpoint the process is repeated.

FIG. 3 is an exemplary timing diagram illustrating ananti-tachyarrhythmia pacing therapy before capacitor charging (ATP-BCC)mode of an implantable medical device according to the presentinvention. As illustrated in FIG. 3, delivery of ATP therapy 204 isinitiated at time 220 following detection of a VT episode 201 andcontinues through a corresponding delivery time 222. In the exampleillustrated in FIG. 3, ATP therapy returns the patient to normal sinusrhythm 210. The ICD device detects the break in VT by the change incardiac rate as well as the return to normal sinus rhythm 210 during aredetection or verification period 224. As a result, no charging of thehigh-voltage capacitors is initiated at time 226. However, if theATP-BCC therapy does not return the patient to normal sinus rhythm andthe VT episode 201 is redetected during verification period 224, anothersequence of ATP therapy is initiated simultaneously with charging of thecapacitors 109, as illustrated in FIG. 4, and described below.

According to the current invention, operation of the ICD may transitionfrom ATP-DCC mode shown in FIGS. 2A and 2B to execution in ATP-BCC modeshown in FIG. 3 based on programmable criteria. In one embodiment, this“Charge Saver” function switches the ICD device operation from ATP-DCCto ATP-BCC mode after attaining a user-programmed consecutive number ofATP successes since the previous follow-up session. ATP therapy isgenerally considered successful when the VT breaks/aborts prior to shockdelivery, although other criteria may be defined for determining thesuccess of the ATP therapy. The device will revert back to ATP-DCC modefollowing a predetermined criteria, which may include a predeterminednumber of failures to break a VT in the ATP-BCC operational mode, aswill be discussed further in reference to FIG. 4.

FIG. 4 is an exemplary timing diagram illustrating an ongoing VT episodethat fails to break following ATP-BCC therapy. As illustrated in FIG. 4,ATP-BCC therapy 204 is delivered during delivery time 232 following VTdetection 201. Thereafter, verification period 233 confirms the ongoingVT episode 201 b. According to an embodiment of the present invention,once it is determined that the initial sequence of ATP-BCC therapy 204was not successful at terminating the VT episode, i.e., at time 236, asecond sequence of ATP therapy 204 b is delivered over a time period 240coinciding with charging of capacitors 236, with both capacitor charging238 and ATP therapy delivery 204 b beginning substantiallysimultaneously at time 236. Studies such as the Medtronic PainFREE R_(x)study have shown that this additional ATP sequence has a low likelihoodof accelerating the ventricular rate, and in fact, has the potential forterminating a VT episode.

Once delivery of subsequent ATP therapy 204 b is completed, i.e., at endtime 247 of time period 240, capacitor charging 238 is paused, and adetermination is made as to whether the VT rhythm 201 is redetectedduring a redetection or verification period 249 during which adetermination is made as to whether the VT rhythm 201 is redetected. If,as illustrated in FIG. 4, VT rhythm 201 is redetected during period 249,capacitor charging 238 is resumed once redetection or verificationperiod 249 is completed, i.e., at end time 242.

According to one embodiment of the present invention, a second sequenceof ATP-DCC therapy may be delivered substantially simultaneously withthe resumption of capacitor charging 238 at end time 242 and the processis repeated until capacitor 109 is completely charged, as will bedescribed in detail below. According to another embodiment of thepresent invention, illustrated in FIG. 4, no additional ATP-DCC therapyis delivered and resumption of capacitor charging 238 continues untilcapacitors 109 are charged to a desired charge level at charge time end251 in preparation for delivery of a shock, if necessary. Anon-committed synchronization period 244 begins once capacitors 109 arecharged to the desired charge level at charge time end 251. During thissynchronization period 244, the patient's cardiac rhythm is evaluated tolocate an appropriate time to deliver a shock and to determine if the VTrhythm is redetected. The shock will be delivered at an end 246 of thesynchronization period 244 unless it is determined duringresynchronization period 244 that the VT episode has terminated. If theepisode is determined to have terminated during synchronization period244, the process charge remains on the capacitors 109 and the devicecontinues to monitor for subsequent detected VT rhythms.

According to another aspect of the present invention, if a predeterminednumber of episodes of VT are not terminated by ATP-BCC therapy such thatshock delivery occurs as shown in FIG. 4, the system reverts fromATP-BCC mode to the ATP-DCC mode.

FIG. 5 is a state diagram illustrating transitions between therapymodes, according to the present invention. ICD devices are shipped fromthe factory with ATP-DCC mode and the Charge Saver feature enabled, asillustrated by state 270 as well as the Charge Saver feature. At thetime of implant, the physician may choose whether to disable the ChargeSaver feature. In one embodiment of the invention, other programmableparameters may be selected by the physician if the Charge Saver featureis enabled. These parameters may include the number of successfulATP-DCC therapy sessions that must be delivered prior to the automatedactivation of ATP-BCC mode, as will be discussed further below. It isunderstood that the ICD device could be shipped with the ATP-DCC modeand the Charge Saver feature disabled so that the physician chooseswhether to enable the features at the time of implant.

During operation with Charge Saver enabled and the system operating inATP-DCC, a transition to ATP-BCC mode shown as state 274 may betriggered by the delivery of a predetermined number X of ATP-DCC therapysessions that succeed in breaking the VT rhythm. This transition isdepicted by arrow 272. Conversely, when operating in ATP-BCC mode andafter a predetermined number Y of failed ATP-BCC therapy attempts, thesystem transitions to ATP-DCC mode as shown by arrow 276. As discussedabove, in one embodiment of the invention, X and Y are programmable.Alternatively, these numbers may be predetermined, non-programmablevalues. Finally, these numbers may represent consecutive ATP therapysessions, or may involve a set of “S of T” therapy sessions. Forexample, a transition from ATP-DCC to ATP-BCC may be selected to occurif 4 of 5 ATP-DCC therapy sessions are determined to be successful.

Other trigger criteria may be used instead of, or in addition to, theabove criteria to initiate a switch between ATP-DCC and ATP-BCC modes.In one embodiment, the system stores both cycle length (CL) and/orR-wave morphology of a VT rhythm to determine whether the type of VTcurrently being experienced is the same type of VT that occurred duringa recently-detected episode or episodes. This is important sincepatients can exhibit different types of VT, each of which may responddifferently to ATP therapy. If the characteristics of the currentepisode are the same as the previous episode, and the previous episoderesponded favorably to ATP-BCC therapy, the device remains in theATP-BCC mode of operation upon detection of a break in rate. On theother hand, if the CL and/or R-wave morphology has changed, the systemmay be programmed to revert back to the ATP-DCC mode of operation.

According to the foregoing embodiment, different mode transitioncriteria may be specified for each type of VT rhythm. For example, atransition from ATP-DCC to ATP-BCC therapy may be triggered by Mconsecutive successful therapy sessions for a first type of VT. Thissame mode transition may be triggered by M′ of N successful therapysessions for a second type of VT. This allows therapies to beindividually selected for different types of VT rhythms.

In yet another embodiment, the system mode-switching criteria takes intoaccount VT frequency. As discussed above, some patients experience “VTstorms” involving the occurrence of a large number of episodes within ashort period of time, such as hours or even minutes. Such episodes,which usually involve VT rhythms having similar CLs and morphologies,may significantly impact battery resources. In this embodiment, theoccurrence of a predetermined number of VT episodes in a predeterminedtime period may trigger a switch from ATP-DCC to ATP-BCC mode to savebattery resources.

According to an alternative embodiment of the invention, a programmablethreshold duration is used to detect VT storms. If two consecutive VTepisodes occur within this predefined threshold duration, a count isincremented. If the count reaches a predetermined value within somelarger programmable time period, a mode switch may be triggered. Once amode switch to ATP-BCC mode occurs, continued operation in ATP-BCC modemay be predicated on obtaining a predetermined success rate using any ofthe mechanisms discussed above. Alternatively, another threshold timecan be defined to track episode frequency in the ATP-BCC mode such thatif the inter-episode duration exceeds this value, a transition back toATP-DCC mode occurs.

If desired, waveform morphology criteria may be applied to VT stormdetection. For example, VT episodes that are separated by longer periodsof time such as weeks or months may involve different types of VTrhythms. Therefore, for all VT episodes, or just the VT episodesseparated by a predetermined time period, mode-switch criteria may beindividually specified for respective types of VT rhythms as discussedabove.

Transition from ATP-DCC to ATP-BCC mode or vice versa could also bepredicated on the length of an episode. For example, the episode lengthmeasured from first detection to the termination of a rhythm could beused as the mode-switching criteria. In one embodiment, longer episodescould trigger a transition to ATP-DCC mode.

According to yet another aspect of the invention, the detection of VTstorms may trigger a patient alert (audible, vibratory or other). Forexample, the patient may be notified to contact a physician so thatoperating parameters of the system may be re-evaluated, andmode-switching conditions may be re-programmed, if necessary.

Another aspect of the invention relates to an optional programmablefeature for disabling all modes of ATP. If this “Smart Mode” feature isenabled and a predetermined criteria is met, all ATP therapy isdisabled. In one embodiment, this Smart Mode feature operates whenexecution is occurring in ATP-DCC mode and a predetermined number offailed therapy attempts is detected. This transition is shown by arrow278 and state 280. The number of failed therapy attempts needed totrigger this transition may be programmable, or may be a predeterminednumber which is preferably “four”. Thereafter, the ICD device will onlydeliver the programmed shock therapy. In another embodiment, thisfeature could also be provided when execution is occurring in ATP-BCCmode, as shown by arrow 281. In yet another embodiment, the switch fromeither ATP-BCC or ATP-DCC mode could be triggered by a VT rhythm orwaveform morphology that meets a predetermined criteria. For instance,the transition to a mode wherein ATP is disabled may be triggered bydetection of a fast VT rhythm that exceeds 250 bpm.

In one embodiment, after a transition occurs to a mode wherein ATP isdisabled, shock therapy will continue until intervention is provided tore-activate the ATP-DCC mode. Such intervention may be provided, forexample, during a subsequent follow-up session. In another embodiment,the system will continue operation in this mode until a defined criteriais met. For example, if the transition to the ATP-disabled mode occursbecause of a fast VT rhythm, the system will revert back to the previousmode of operation after the fast VT episode has been terminated by theshock delivery, as shown by arrows 283 and 284.

FIG. 6A is a flowchart of operation of an implantable medical device inan ATP-DCC mode, according to an embodiment of the present invention. Asillustrated in FIG. 6A, an implantable medical device such as the oneshown in FIG. 1 is generally implanted with the ATP-DCC mode enabled,although it maybe implanted with ATP-BCC mode enabled, if desired. Whilein the nominal ATP-DCC mode, block 350, the device continuously monitorsfor the presence of tachyarrhythmias. Once a VT rhythm is detected,block 352, for example, delivery of an initial ATP-DCC therapy sequenceand charging of the high-voltage capacitors are initiated substantiallysimultaneously, block 354. After delivery of the initial ATP-DCC therapysequence is completed, YES in block 356, a determination is made as towhether charging of the capacitors 109 is completed to a desired level,block 374.

If the capacitors 109 are not yet charged to the desired charge level,NO in block 374, charging of the capacitors 109 is paused, block 376,and a determination is made as to whether the initially delivered ATPsequence was successful at terminating the VT rhythm, block 378. If theinitial ATP sequence was successful at terminating the VT rhythm and therhythm is not redetected, NO in block 378, a determination is made as towhether the device should transition from the ATP-DCC mode to theATP-BCC mode, block 372, based on the factors described above inreference to FIG. 5. If it is determined that the device should nottransition from the ATP-DCC mode to the ATP-BCC mode, NO in block 372,the process returns to block 352 to monitor for subsequent detected VTrhythms, at which point the process is repeated. If a mode switch isindicated, YES in block 372, the device transitions to the ATP-BCC mode,block 450, which is described below in reference to FIG. 6B.

If the initially ATP sequence was not successful at terminating the VTrhythm and the rhythm is redetected, YES in block 378, charging of thecapacitors 109 is resumed, block 380, and a determination is made as towhether another ATP sequence should be initiated prior to delivering ofshock therapy, block 358.

According to the present invention, the number of ATP sequences that aredelivered prior to delivering shock therapy is programmable, and caninclude only one initial sequence, or a multiple number of sequences,such as three for example. The number chosen may be dependent upon manyfactors or combination of factors, such as the rate of the detectedrhythm, whether the detected rhythm is a stable rhythm, or whether thedetected rhythm is part of a cluster of detected rhythms that occur in aspecified period of time.

If the programmed number of ATP sequences have been delivered, NO inblock 358, and capacitors 109 are charged to the desired charge level,YES in block 360, a non-committed synchronization period begins duringwhich the patient's cardiac rhythm is evaluated to locate an appropriatetime to deliver a shock, block 364, and to determine if the VT rhythm isredetected, block 362. The shock will be delivered, block 366, at an endof the synchronization period unless it is determined that the VTepisode has terminated, i.e., is no longer detected, NO in block 362.

If the episode is no longer detected, NO in block 362, the determinationis made as to whether the device should transition from the ATP-DCC modeto the ATP-BCC mode, block 372, based on the factors described above inreference to FIG. 5. If it is determined that the device should nottransition from the ATP-DCC mode to the ATP-BCC mode, NO in block 372,the process returns to block 352 to monitor for subsequent detected VTrhythms, at which point the process is repeated. If a mode switch isindicated, YES in block 372, the device transitions to the ATP-BCC mode,block 450, which is described below in reference to FIG. 6B.

Once the synchronization period is completed, YES in block 364, theshock is delivered, block 366. Upon completion of delivery of the shocktherapy, a determination is made as to whether the VT rhythm wasterminated by the delivered shock, block 368. Several criteria may beused to make this determination, including cardiac rate, cycle length,R-wave morphology, and/or any other criteria known in the art for thispurpose. If the VT has not terminated, the device begins the process ofdelivering a next programmed therapy in a tiered therapy approach,assuming a tiered therapy approach is utilized, block 370. Once all ofthe programmed therapies have been exhausted in block 370, or in thecase where a tiered approach is not utilized after the shock wasdelivered in block 366, or if the VT rhythm is not redetected afterdelivery of the shock NO in block 368, the determination is made as towhether the device should transition from the ATP-DCC mode to theATP-BCC mode, block 372, based on the factors described above inreference to FIG. 5. If it is determined that the device should nottransition from the ATP-DCC mode to the ATP-BCC mode, NO in block 372,the process returns to block 352 to monitor for subsequent detected VTrhythms, at which point the process is repeated. If a mode switch isindicated, YES in block 372, the device transitions to the ATP-BCC mode,block 450, which is described below in reference to FIG. 6B.

If it is determined that all of the predetermined number of ATP therapysequences have not been delivered, i.e., another ATP sequence should bedelivered prior to delivering shock therapy, YES in block 358, thesubsequent sequence of ATP therapy is delivered, block 382. Oncedelivery of the subsequent ATP therapy has completed, charging ofcapacitors 109 is paused, block 384, and a determination is made as towhether the subsequent delivered ATP sequence was successful atterminating the VT rhythm, block 386. If the VT rhythm was notterminated and is redetected, YES in block 386, charging of capacitors109 is resumed, block 380, and the above-described process ofdetermining whether the programmed number of ATP sequences have beendelivered, block 358, is repeated.

If the VT rhythm was terminated as a result of the last delivered ATPsequence and is not redetected, NO in block 386, the determination ismade as to whether the device should transition from the ATP-DCC mode tothe ATP-BCC mode, block 372, based on the factors described above inreference to FIG. 5. If it is determined that the device should nottransition from the ATP-DCC mode to the ATP-BCC mode, NO in block 372,the process returns to block 352 to monitor for subsequent detected VTrhythms, at which point the process is repeated. If a mode switch isindicated, YES in block 372, the device transitions to the ATP-BCC mode,block 450, which is described below in reference to FIG. 6B.

If capacitors 109 are determined to be charged to a desired charge levelonce delivery of the initial ATP-DCC therapy sequence is completed, YESin block 374, a determination is made as to whether the deliveredinitial ATP-DCC therapy sequence was successful at terminating the VTrhythm, block 375. If the initial ATP-DCC therapy sequence wassuccessful at terminating the VT rhythm and therefore the rhythm is notredetected, NO in block 375, the determination is made as to whether thedevice should transition from the ATP-DCC mode to the ATP-BCC mode,block 372, based on the factors described above in reference to FIG. 5.If it is determined that the device should not transition from theATP-DCC mode to the ATP-BCC mode, NO in block 372, the process returnsto block 352 to monitor for subsequent detected VT rhythms, at whichpoint the process is repeated. If a mode switch is indicated, YES inblock 372, the device transitions to the ATP-BCC mode, block 450, whichis described below in reference to FIG. 6B.

If capacitor charging has completed and the initial ATP-DCC therapysequence was not successful at terminating the VT rhythm and thereforethe rhythm redetected, YES in blocks 374 and 375, the above-describedprocess of determining whether the programmed number of ATP sequenceshave been delivered, block 358, described above, is repeated, and istherefore omitted for the sake of brevity.

FIG. 6B is a flowchart of operation of an implantable medical device inan ATP-BCC mode, according to an embodiment of the present invention. Asillustrated in FIG. 6B, when in the ATP-BCC mode state of operation,block 450, the device monitors for the presence of tachyarrhythmias,block 452. When a tachyarrhythmia meets VT criteria, for example, YES inblock 452, an ATP-BCC therapy sequence is initiated without initiatingcharging of the capacitors 109, block 454. Once delivery of the ATP-BCCtherapy sequence has completed, a determination is made as to whetherthe delivered ATP-DCC therapy sequence was successful at terminating theVT rhythm, block 460. If ATP-BCC therapy has successfully terminated theVT rhythm, NO in block 460, the system reverts to the nominal stateillustrated in block 450.

If the ATP-BCC therapy was not successful and the VT episode isredetected and meets VT criteria, YES in block 460, a determination ismade as to whether another sequence of the ATP-BCC therapy should bedelivered, block 461. According to the present invention, the number ofATP sequences that may be delivered prior to initiating the charging ofhigh voltage capacitors and delivery of the ATP-DCC therapy of block 462is programmable, and can include only a single sequence, or a multiplenumber of sequences, such as three for example. The number chosen may bedependent upon many factors or combination of factors, such as the rateof the detected rhythm, whether the detected rhythm is a stable rhythm,or whether the detected rhythm is part of a cluster of detected rhythmsthat occur in a specified period of time.

Once the programmed number of ATP-BCC sequences have been delivered, NOin block 461, delivery of an ATP-DCC therapy sequence and charging ofthe high-voltage capacitors are initiated substantially simultaneously,block 462. After delivery of the initial ATP-DCC therapy sequence hascompleted, YES in block 482, a determination is made as to whethercharging of the capacitors is completed to a desired level, block 464.If capacitors 109 are not yet charged to a desired charge level, NO inblock 464, charging of the capacitors 109 is paused, block 484, and adetermination is made as to whether the initially delivered ATP sequencewas successful at terminating the VT rhythm, block 486.

If the initially delivered ATP sequence was successful and therefore theepisode is no longer detected, NO in block 486, a determination is madeas to whether the device should transition from the ATP-BCC mode to theATP-DCC mode, block 480, based on the factors described above inreference to FIG. 5. If it is determined that the device should nottransition from the ATP-BCC mode to the ATP-DCC mode, NO in block 480,the process returns to block 452 to monitor for subsequent detected VTrhythms, at which point the process is repeated. If a mode switch isindicated, YES in block 480, the device transitions to the ATP-DCC mode,block 350, which is described above in reference to FIG. 6A.

If the initially delivered ATP sequence was not successful and thereforethe episode is redetected, YES in block 486, charging of the capacitorsis resumed, block 488, and a determination is made as to whether anotherATP sequence should be initiated prior to delivering of shock therapy,block 466. According to the present invention, the number of subsequentATP sequences that may be delivered after delivery of the initialsequence and prior to delivering the shock therapy is programmable, andcan include only a single additional sequence, or a multiple number ofsequences. The number chosen may be dependent upon many factors orcombination of factors, such as the rate of the detected rhythm, whetherthe detected rhythm is a stable rhythm, or whether the detected rhythmis part of a cluster of detected rhythms that occur in a specifiedperiod of time.

Once a subsequent ATP-DCC sequence is delivered, block 490, charging ofcapacitors 109 is paused, block 492, and a determination is made as towhether the subsequent delivered ATP sequence or sequences wassuccessful at terminating the VT rhythm, block 494. If the VT rhythm wasnot terminated and is redetected, YES in block 494, charging ofcapacitors 109 is resumed, block 488, and the above-described process ofdetermining whether another ATP sequences should be delivered, block466, is repeated. If the VT rhythm is terminated by the subsequentdelivered ATP sequence and therefore is not redetected, NO in block 494,the determination is made as to whether the device should transitionfrom the ATP-BCC mode to the ATP-DCC mode, block 480, based on thefactors described above in reference to FIG. 5. If it is determined thatthe device should not transition from the ATP-BCC mode to the ATP-DCCmode, NO in block 480, the process returns to block 452 to monitor forsubsequent detected VT rhythms, at which point the process is repeated.If a mode switch is indicated, YES in block 480, the device transitionsto the ATP-DCC mode, block 350, which is described above in reference toFIG. 6A.

Once the programmed number of ATP sequences have been delivered, NO inblock 466, and capacitors 109 are charged to the desired charge level,YES in block 468, the non-committed synchronization period begins duringwhich the patient's cardiac rhythm is evaluated to locate an appropriatetime to deliver a shock, block 472, and to determine if the VT rhythm isredetected, block 470. The shock will be delivered, block 474, at an endof the synchronization period unless it is determined that the VTepisode has terminated and is no longer detected, NO in block 470. Ifthe episode is no longer detected, a determination is made as to whetherthe device should transition from the ATP-BCC mode to the ATP-DCC mode,block 480, based on the factors described above in reference to FIG. 5.If it is determined that the device should not transition from theATP-BCC mode to the ATP-DCC mode, NO in block 480, the process returnsto block 452 to monitor for subsequent detected VT rhythms, at whichpoint the process is repeated. If a mode switch is indicated, YES inblock 480, the device transitions to the ATP-DCC mode, block 350, whichis described above in reference to FIG. 6A.

After the shock has been delivered in block 474, a determination is madeas to whether the VT rhythm was terminated by the delivered shock, block476. Several criteria may be used to make this determination, includingcardiac rate, cycle length, R-wave morphology, and/or any other criteriaknown in the art for this purpose. If the VT rhythm has not terminated,the device begins the process of delivering a next programmed therapy ina tiered therapy approach, assuming a tiered therapy approach isutilized, block 478. Once all of the programmed therapies have beenexhausted in block 478, or in the case where a tiered approach is notutilized, once the shock is delivered in block 474, or if the VT rhythmis no longer detected after delivery of the shock, NO in block 476, adetermination is made as to whether the device should transition fromthe ATP-BCC mode to the ATP-DCC mode, block 480, based on the factorsdescribed above in reference to FIG. 5. If it is determined that thedevice should not transition from the ATP-BCC mode to the ATP-DCC mode,NO in block 480, the process returns to block 452 to monitor forsubsequent detected VT rhythms, at which point the process is repeated.If a mode switch is indicated, YES in block 480, the device transitionsto the ATP-DCC mode, block 350, which is described above in reference toFIG. 6A.

If capacitors 109 are determined to be charged to a desired charge levelonce delivery of the initial ATP-DCC therapy sequence is completed, YESin block 464, a determination is made as to whether the deliveredinitial ATP-DCC therapy sequence was successful at terminating the VTrhythm, block 475. If the initial ATP-DCC therapy sequence wassuccessful at terminating the VT rhythm and therefore the rhythm is notredetected, NO in block 475, the determination is made as to whether thedevice should transition from the ATP-DCC mode to the ATP-BCC mode,block 480, based on the factors described above in reference to FIG. 5.If it is determined that the device should not transition from theATP-DCC mode to the ATP-BCC mode, NO in block 480, the process returnsto block 452 to monitor for subsequent detected VT rhythms, at whichpoint the process is repeated. If a mode switch is indicated, YES inblock 480, the device transitions to the ATP-DCC mode, block 350, whichis described below in reference to FIG. 6A.

If capacitor charging has completed and the initial ATP-DCC therapysequence was not successful at terminating the VT rhythm and thereforethe rhythm is redetected, YES in blocks 464 and 475, the above-describedprocess of determining whether the programmed number of ATP sequenceshave been delivered, block 466, described above, is repeated. Since theprocess of determining whether the programmed number of ATP sequenceshave been delivered, block 466, is described above, the descriptionherein is omitted merely for the sake of brevity.

FIG. 6C is a flowchart of operation of an implantable medical device ina best available therapy mode, according to an embodiment of the presentinvention. According to an embodiment of the present invention, inaddition to having the capability of programming the number ofsubsequent ATP sequences that may be delivered after delivery of theinitial sequence and prior to delivering the shock therapy, theimplantable medical device may also include the programmably selectedoption of utilizing a best available therapy mode. The best availabletherapy mode provides continuous delivery of low-energy therapy untilcapacitor charge makes the high-energy therapy available. For example,as illustrated in FIGS. 6A-6C, while in either the ATP-DCC mode or theATP-BCC mode, once the initial ATP sequence has been delivered, block356, 482, and the programmed number of subsequent ATP sequences havebeen delivered, NO in block 358, 466, if the capacitors are not chargedto the desired level for discharge necessary for deliver of the shocktherapy, NO in block 360, 468, another ATP sequence is delivered, block502. Once the additional ATP sequence has completed, YES in block 506,and as long as the capacitors are not yet charged to the dischargelevel, NO in block 504, a determination is made as to whether theadditional delivered ATP sequence was successful in terminating therhythm, block 508.

If the additional ATP sequence delivered in block 502 was successful atterminating the rhythm and therefore the rhythm is not redetected, NO inblock 508, the determination is made as to whether the device shouldtransition from the ATP-DCC mode to the ATP-BCC mode, or vice versa,depending upon the mode the device is currently in, blocks 372 and 480,as described above. If the ATP sequence delivered in block 502 was notsuccessful and therefore the rhythm is redetected, YES in block 508,another ATP sequence beyond the predetermined number of sequencesdelivered in block 358, 466 is again delivered, block 502, unless thecapacitors are determined to be charged to the discharge level, YES inblock 360, 468, subsequent to delivery of the additional ATP sequence orsequences. Once the capacitors are determined to be charged to thedischarge level, either during delivery of a subsequent additional ATPsequence or sequences, block 504, or subsequent to the delivery of anadditional ATP sequence, YES in block 360, 468, delivery of theadditional ATP therapy is aborted, block 510, and shock therapy isdelivered. According to an embodiment of the present invention, deliveryof shock therapy during the best available therapy mode of operationoccurs after synchronizing the delivery of the shock therapy and adetermination is made as to whether the VT rhythm is redetected, asdescribed above, so that the shock therapy will be delivered at the endof the synchronization period unless it is determined that the VT rhythmhas terminated. According to an embodiment of the present invention, asshown in FIG. 6C, delivery of the shock therapy while the device is inthe best available therapy mode of operation is expedited by notincluding performing redetection of the VT rhythm once the capacitorsare charged to the discharge level, Rather, once the capacitors arecharged to the desired discharge level, Yes in block 504, the ATPtherapy is aborted, block 510, and the shock therapy is deliveredimmediately upon completion of synchronization, block 512, withoutperforming redetection.

In this way, the device continues to deliver ATP therapy during allavailable ATP therapy delivery opportunities, particularly during thoseinstances where the amount of time required to charge the capacitors tothe desired discharge level increases, for example. In addition, byenabling ATP therapy to continue to be delivered throughout thecapacitor charge time, the present invention increases the possibilityof reducing the necessity for delivery of shock therapy, such as in thecase where the subsequently delivered additional ATP sequence, block502, is successful at terminating the rhythm before the capacitors arecharged to the discharge level.

FIG. 6D is a flowchart of operation of an implantable medical device ina best available therapy mode, according to an embodiment of the presentinvention. According to an embodiment of the present invention, pausingof the charging of the capacitors, as described above, may be utilizedin the best available therapy mode of operation. For example, asillustrated in FIG. 6D, once delivery of the additional ATP sequenceblock 502 is completed, Yes in block 506, and as long as the capacitorsare not charged to the discharge level, NO in block 504, charging of thecapacitors may be paused, block 514, during the determination as towhether the additional delivered ATP sequence was successful interminating the rhythm, block 508. If the additional ATP sequence wasnot successful, charging of the capacitors is resumed, block 516, andanother ATP sequence beyond the predetermined number of sequencesdelivered in block 358, 466 is again delivered, block 502, unless thecapacitors are determined to be charged to the discharge level, YES inblock 360, 468, subsequent to delivery of the additional ATP sequence orsequences, as described above.

FIG. 7 is a flowchart of a mode switch according to an embodiment of thepresent invention. As illustrated in FIG. 7, in order to determinewhether to transition from the ATP-DCC mode to the ATP-BCC mode in block372 of FIG. 6A, a count of successful ATP therapy sessions isincremented each time the most-recently provided ATP therapy terminatesthe VT rhythm, block 550. A determination is then made as to whetherrhythm-specific criteria will be used to make the mode-switchdetermination, block 552. As described above, it may be desirable todefine specific criteria for the various types of VT rhythms, as may beidentified by cycle length, and waveform morphology.

If rhythm-specific criteria will be utilized, the VT rhythm associatedwith the most recent VT episode is analyzed, and the correspondingcriteria retrieved, as shown in block 554. Otherwise, the standardcriterion is utilized. This criterion may be programmable, or a pre-setvalue.

After the criterion is selected, if necessary, the count of successfulATP therapy sessions is compared against the appropriate criteria inblock 556 to determine whether a mode switch should be performed. It maybe noted that this criteria may involve a consecutive number ofsuccesses, a predetermined number of successes in a predetermined periodof time, or may instead require X of Y successes, as discussed above.Other criteria that do, or do not, involve a count of successfultherapy-delivery sessions may be used instead of, or in addition to, thepredetermined count criteria. For example, the duration of a VT episodemay be utilized to trigger a mode switch to ATP-BCC mode, if desired. Aswill be discussed further below, this criteria may includepatient-specific criteria. If the pre-defined criteria are met, the modeswitch from the ATP-DCC mode, block 350, to the ATP-BCC mode, block 450,is performed, block 558.

If the predetermined criteria are not met in decision block 556, adetermination is made as to whether VT-frequency monitoring is enabled,block 560 so that VT storms may be detected. If VT-frequency monitoringis enabled, a determination is made as to whether the VT-frequencycriteria are met, block 562. This involves making a determination as towhether a predetermined number of VT episodes are detected in a specificperiod of time. Alternatively, an inter-episode threshold duration maybe defined to detect VT storms in the manner discussed above. Thedetection may also take into consideration types of VT episodes, ifdesired. For example, separate running counts may be maintained forvarious types of VT episodes, with the types being determined by CL andwaveform morphology. Each type of episode may also be associated withdifferent criteria in a manner similar to that discussed. For example, aVT storm indication may be met if a first type of VT episode occurs Xtimes in Y minutes, whereas a VT storm indication is met for a secondtype of VT episode occurring X′ times in Y′ minutes, and so on.

If any of the one or more VT-frequency criteria is met, a mode switchfrom ATP-DCC therapy mode to ATP-BCC therapy mode occurs, block 558, andprocessing continues in ATP-BCC mode, block 450 of FIG. 6B. Otherwise,if VT-frequency detection is not enabled, or the VT-frequency criteriaare not met, no mode switch occurs, block 564, and processing continuesin ATP-DCC mode, block 350 of FIG. 6A.

FIG. 8 is a flowchart illustrating a mode switch according to anembodiment of the present invention. As illustrated in FIG. 8, in orderto determine whether to transition from the ATP-BCC mode to the ATP-DCCmode in block 480 of FIG. 6B, a count of unsuccessful ATP therapysessions is incremented each time that the most-recently provided ATPtherapy fails to terminate the VT rhythm, block 650. A determination isthen made as to whether rhythm-specific criteria will be used to makethe mode-switch determination, block 652. As described above, differentcriteria may be defined for different VT rhythms.

If rhythm-specific criteria will be utilized, the VT rhythm associatedwith the most recent VT episode is analyzed, and the correspondingcriteria retrieved, as shown in block 654. Such rhythm-specific criteriamay involve a mode switch from ATP-BCC to ATP-DCC mode based on thedetection of a particular type of VT episode, for instance. In anotherinstance, the rhythm-specific criteria may involve a count of a numberof failed therapy attempts, for example.

If rhythm-specific criteria are not to be utilized as determined inblock 652, a standard criterion may be utilized. In either case, theappropriate criteria are used in block 656 to determine whether a modeswitch from the ATP-BCC therapy mode to the ATP-DCC therapy mode shouldbe performed. It may be noted that this criteria may involve aconsecutive number of failed therapy attempts, may instead require X ofY failed therapy attempts, or may require a predetermined number offailures in a predetermined amount of time as discussed above. In oneembodiment, a predetermined number of failed therapy attempts from thelast patient medical check-up may be utilized as the trigger criteria.In another embodiment, the criteria may alternatively or additionallyinclude conditions unrelated to failed therapy attempts, such as theoccurrence of a particular type of rhythm, or a specific change in atype of rhythm, as noted above. This criteria may also includepatient-specific conditions related to patient medical history. If thiscriteria is met, YES in block 656, the mode switch is performed, block658, and processing continues in ATP-DCC therapy mode, block 350 of FIG.6A. If the criteria are not met, no mode switch is performed, block 564,and processing continues in the ATP-BCC therapy mode, block 450 of FIG.6B.

As discussed above, many different types of criteria may be used totrigger a mode switch. In one embodiment, this criteria is programmable,and may be initially programmed and/or thereafter altered based onpatient history. This allows system operation to be tailored for eachpatient. This could take into account, for example, a patient'sindividual response to ATP therapies. Programming can be accomplished,for example, using telemetry systems known in the art.

Some of the techniques described above may be embodied as acomputer-readable medium comprising instructions for a programmableprocessor such as microprocessor 100 or control circuitry 106 shown inFIG. 1. The programmable processor may include one or more individualprocessors, which may act independently or in concert. A“computer-readable medium” includes but is not limited to any type ofcomputer memory such as floppy disks, conventional hard disks, CR-ROMS,Flash ROMS, nonvolatile ROMS, RAM and a magnetic or optical storagemedium. The medium may include instructions for causing a processor toperform any of the features described above for initiating a session ofthe escape rate variation according to the present invention.

The preceding specific embodiments are illustrative of the practice ofthe invention. It is to be understood, therefore, that other expedientsknown to those of skill in the art or disclosed herein may be employedwithout departing from the invention or the scope of the appended claim.It is therefore to be understood that the invention may be practicedotherwise than as specifically described, without departing from thescope of the present invention. As to every element, it may be replacedby any one of infinite equivalent alternatives, only some of which aredisclosed in the specification.

1. An implantable medical device, comprising: means for sensing cardiacsignals; means for detecting a predetermined event in response to thesensed signals; means for delivering a first therapy during a firstdelivery period, substantially simultaneous with coupling of a chargingcircuit and an energy storage device to generate stored energy on theenergy storage device, in response to the predetermined event beingdetected; means for determining whether to deliver the first therapyduring a second delivery period subsequent to the first delivery period;means for determining whether there is a predetermined level of storedenergy generated on the energy storage device; and means for delivering,in response to the first therapy not being delivered during the seconddelivery period and the predetermined level of stored energy not beinggenerated on the energy storage device, the first therapy during a thirddelivery period subsequent to the second delivery period.
 2. The deviceof claim 1, further comprising: means for decoupling the chargingcircuit and energy storage device during a redetect period subsequent todelivery of the first therapy during the third delivery period; andmeans for recoupling the energy storage device and the charging circuitin response to detecting the predetermined event during the redetectperiod.
 3. The device of claim 1, further comprising means for abortingdeliver of the first therapy during the third delivery period inresponse to the predetermined level of stored energy being generated onthe energy storage device.
 4. The device of claim 1, further comprising:means for delivering a second therapy associated with the predeterminedlevel of stored energy; and means for determining a location fordelivery of the second therapy in response to the sensed cardiacsignals, wherein delivery of the second therapy is substantiallysimultaneous with the determination of the location and does not includea redetect period.
 5. The device of claim 1, further comprising: meansfor decoupling the charging circuit and energy storage device during aredetect period subsequent to delivery of the first therapy; and meansfor recoupling the energy storage device and the charging circuit inresponse to detecting the predetermined event during the redetectperiod.
 6. The device of claim 1, further comprising: means foroperating in a first mode to couple the charging circuit and the energystorage device during delivery of the first therapy; means for operatingin a second mode to couple the charging circuit and the energy storagedevice subsequent to delivery of the first therapy; and means fortransitioning between the first mode and the second mode based onpredetermined criteria corresponding to the effectiveness of apreviously-delivered first therapy.
 7. A method for delivering therapyin an implantable medical device, comprising: sensing cardiac signals;detecting a predetermined event in response to the sensed signals;delivering a first therapy during a first delivery period, substantiallysimultaneous with coupling of a charging circuit and an energy storagedevice to generate stored energy on the energy storage device, inresponse to the predetermined event being detected; determining whetherto deliver the first therapy during a second delivery period subsequentto the first delivery period in response to the predetermined level ofstored energy not being generated on the energy storage device;determining whether there is a predetermined level of stored energygenerated on the energy storage device; and delivering, in response tothe first therapy not being delivered during the second delivery periodand the predetermined level of stored energy not being generated on theenergy storage device, the first therapy during a third delivery periodsubsequent to the second delivery period.
 8. The method of claim 7,further comprising: decoupling the charging circuit and energy storagedevice during a redetect period subsequent to delivery of the firsttherapy during the third delivery period; and recoupling the energystorage device and the charging circuit in response to detecting thepredetermined event during the redetect period.
 9. The method of claim7, further comprising aborting deliver of the first therapy during thethird delivery period in response to the predetermined level of storedenergy being generated on the energy storage device.
 10. The method ofclaim 7, further comprising: delivering a second therapy associated withthe predetermined level of stored energy; and determining a location fordelivery of the second therapy in response to the sensed cardiacsignals, wherein delivery of the second therapy is substantiallysimultaneous with the determination of the location and does not includea redetect period.
 11. The method of claim 7, further comprising:decoupling the charging circuit and energy storage device during aredetect period subsequent to delivery of the first therapy; andrecoupling the energy storage device and the charging circuit inresponse to detecting the predetermined event during the redetectperiod.
 12. The method of claim 11, further comprising: operating in afirst mode to couple the charging circuit and the energy storage deviceduring delivery of the first therapy; operating in a second mode tocouple the charging circuit and the energy storage device subsequent todelivery of the first therapy; and transitioning between the first modeand the second mode based on predetermined criteria corresponding to theeffectiveness of a previously-delivered first therapy.
 13. The method ofclaim 12, wherein the predetermined criteria corresponds to a firstparameter associated with a number of times the predetermined event isdetected during the redetect period while operating in the second mode.14. The method of claim 13, wherein the first parameter corresponds to anumber of delivered sequences of the first therapy that do not terminatethe predetermined event out of a total number of delivered sequences ofthe first therapy.
 15. The method of claim 13, wherein the predeterminedcriteria corresponds to a second parameter associated with a number oftimes the predetermined event is detected during the redetect periodwhile operating in the first mode.
 16. The method of claim 15, whereinthe second parameter corresponds to a number of delivered sequences ofthe first therapy that terminate the predetermined event out of a totalnumber of delivered sequences of the first therapy.
 17. The method ofclaim 15, further comprising: determining rhythms associated with thesensed signals; and utilizing different values for the first parameterand the second parameter, each of the values being respectivelyassociated with a rhythm of the determined rhythms occurring duringdelivery of the first therapy.
 18. The method of claim 12, wherein thepredetermined criteria is programmably selected to be specific to agiven patient.
 19. The method of claim 12, further comprisingdetermining rhythms associated with the sensed signals, wherein thepredetermined criteria corresponds to a length of one or more of thedetermined rhythms.
 20. The method of claim 12, further comprisingdetermining rhythms associated with the sensed signals and transitioningfrom the first mode to the second mode in response to a number of apredetermined rhythm of the determined rhythms being detected within apredetermined period of time.
 21. The method of claim 12, wherein thepredetermined criteria includes criteria associated with a change in atype of cardiac rhythm occurring prior to the delivery of the firsttherapy.
 22. The method of claim 21, further comprising transitioningfrom the second mode to the first mode in response to the criteriaassociated with a change in a type of cardiac rhythm occurring duringthe delivery of the first therapy.
 23. The method of claim 12, furthercomprising transitioning between the first mode and the second mode inresponse to a length of an episode corresponding to the delivered firsttherapy.
 24. A computer readable medium having computer executableinstructions for performing a method comprising: sensing cardiacsignals; detecting a predetermined event in response to the sensedsignals; delivering a first therapy during a first delivery period,substantially simultaneous with coupling of a charging circuit and anenergy storage device to generate stored energy on the energy storagedevice, in response to the predetermined event being detected;determining whether to deliver the first therapy during a seconddelivery period subsequent to the first delivery period in response tothe predetermined level of stored energy not being generated on theenergy storage device; determining whether there is a predeterminedlevel of stored energy generated on the energy storage device; anddelivering, in response to the first therapy not being delivered duringthe second delivery period and the predetermined level of stored energynot being generated on the energy storage device, the first therapyduring a third delivery period subsequent to the second delivery period.25. An implantable medical device, comprising: an input circuit sensingcardiac signals; a microprocessor detecting a predetermined event inresponse to the sensed signals; a first circuit delivering a firsttherapy a predetermined number of times; a second circuit for deliveringa second therapy, the second circuit including an energy storage deviceto store energy associated with the second therapy and a chargingcircuit selectively coupled to the energy storage device to generate thestored energy; and a control circuit controlling the first circuit andthe second circuit to deliver the first therapy substantiallysimultaneous with coupling of the charging circuit and the energystorage device in response to the predetermined event being detected,the control circuit decoupling the energy storage device and thecharging circuit during a redetect period subsequent to delivery of thefirst therapy, and recoupling the energy storage device and the chargingcircuit in response to the microprocessor detecting the predeterminedevent during the redetect period, wherein the control circuit deliversthe first therapy subsequent to the first therapy being delivered thepredetermined number of times in response to a predetermined level ofstored energy not being generated on the energy storage device.